Wet strength paper and process for the production thereof



Patented May 6, 1952 WET STRENGTH PAPER AND PROCESS FOR THE PRODUCTION THEREOF John H. Daniel, Jr., Stamford, and Chester G.

Landes, New Canaan, Conn., assignors to American Cyanamid Company, New York, N. Y., a

corporation of Maine No Drawing. Application August 3, 1946, Serial No. 688,334

This invention relates to the manufacture of resin-treated cellulosic fibers and fibrous ma-- terials prepared therefrom, and is directed particularly to a method for improving the wet strength of felted fibrous cellulosic materialssuch as paper, board, shaped paper articles and the like. The invention includes the improved cellulosic fibers and fibrous products themselves as Well as methods of preparing these products from aqueous suspensions of fibrous cellulosic materials such as paper pulp.

It has been known for some time that the wet tensile strength and the bursting strength of paper can be increased by soaking the formed paper in strong solutions of urea-formaldehyde resin, followed by heating the paper to evaporate the water and cure the resin. In some paper mills the urea-formaldehyde resin solution has been applied byspraying the solution onto a moving web of paper, followed by passing the paper over or between heated drying rolls. However, a number of practical objections have developed to this method of treatment, the most serious of which is that the evaporation of the additional water introduced with the resin requires a second heating of the paper if it has first been dried, or, if the paper is impregnated while it is still wet, a material reduction in the speed of the drying drums. Despite these objections, however, the so-called tub treatment of preformed paper with concentrated aqueous solutions of synthetic resins is still used in some paper mills for special purposes, and the thermosetting resins hereinafter described may be applied by this method within the broader scope of the present invention if desired.

In order to avoid the difficulties inherent in tub treatment, more recent practice in most paper mills manufacturing wet strength paper has been to apply a melamine-aldehyde resin of a special type, known as colloidal cationic melamine aldehyde resin. The discovery that this type of resin possesses substantive properties for hydrated paper stock, and can be applied .to dilute paper pulp suspensions in small quantities with a high degree of retention and excellent wet strength in the finished paper, was made jointly with Charles S. Maxwell by one of the present applicants. The details of this method of producing wet strength paper are described in an article in the August 9,

15 Claims. (01. 92, s)

2 1945 issue of the Paper Trade Journal. Briefly, melamine-formaldehyde resin is dissolved in a water solution of hydrochloric acid or another strong acid other than sulfuric acid to form a drated paper stock in the beater, stock chest,

Jordan engine, head box or at any other suitable point ahead of the papermaking wire or screen. The stock is then formed into paper by the usual procedure and carried over steam-heated drying rolls which dry the paper and cure the resin to a water-insoluble condition.

It is a principal object of the present invention to provide papermaking fibers and paper impregnated with a thermosetting resin having the properties of imparting wet strength thereto, and also imparting increased dry strength, which resin can be prepared and added to the slush stock in a paper mill in a slightly acid, neutral or alkaline condition. A further object is the provision of a resin of this character having a decreased sensitivity to the presence of large amounts of sulfate ion in the water containing the cellulose fibers. Astill further important object is the provision of a thermosetting resin that can be cured under alkaline conditions, and which can therefore be readily used in the resin sizing of alkaline papers such as those containing calcium carbonate sizes and fillers. Still further objects will become apparent from the following descriptions of preferred embodiments of the invention.

We have found that the above and other objects are accomplished by applying to fibrous cellulosic material such as paper pulp or preformed and dried or partially dried paper an uncured thermosetting resin obtainable by condensing an alkylenepolyamine with a halohydrin, as will hereinafter be more fully described. The resins of this class possess the property of imparting wet strength to paper when applied thereto in amounts on the order of 0.1% to 5% or more, based on the dry weight of the paper. We have also found, as one of the most important features of our invention, that the uncured thermosetting resins of the above class are substantive to fibers of hydrated cellulosic material such as paper pulp in aqueous solution; i. e. the resin is selectively adsorbed or absorbed by the cellulose fibers from a dilute aqueous solution or dispersion thereof containing these fibers in amounts much greater than those corresponding to the concentration of resin in the solution or to what would be contained in the water normally left in the sheet after forming. The importance of this discovery is evident, for it permits the application to cellulosic fibers of sufficient quantities of the resin to impart wet strength while the fibers are in dilute aqueous suspensions of the consistency used in paper mills, which is about 0.1 to 1% or, in pulp molding processes, at higher consistencies up to 22.5%.

The alkylenepolyamines used in preparing the resins employed in practicing our invention are well-known compounds corresponding to the formula H2N(C1LH21L.HN)$H in which is one or more. Typical amines of this class are the alkyl-- enediamines such as ethylenediamine and 1,3- propylenediamine and polyalkylenepolyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine and the corresponding polypropylenepolyamines and polybutylenepolyamines. The halohydrinsare derivatives of glycerol in which one terminal hydroxy group is substituted by a halogen atom; i. e., by chlorine, fluorine, bromine or iodine, representative compounds being alphadichlorhydrin, epichlorhydrin and the like. We have found that the presence of a terminal halogen atom in these reagents imparts cationic properties to the resins which they form by reaction with alkylenepolyamines, and this is probably the reason why these resins are substantive to hydrated cellulose fibers.

In order to obtain a condensation product capable of cross-polymerization to form a resin a halohydrin shOuld be used having at least two groups or radicals capable of promoting combination with an alkylenepolyamine. Such a compound is called by resin chemists a bifunctional or poly-functional halohydrin. The second combining radical may be another halogen atom, as in the dichlorhydrins or dibromhydrins or it may be an epoxide ring as in epichlorhydrin, or any other atom or group that is reactive to a polyamine. This type of poly-functional halohydrin is believed to react with a polyamine in two stages, the first being a simple linear condensation and the second a polymerization or self-alkylation reaction. The condensation between tetraethylenepentamine and epichlorhydrin is typical, and is believed to be as follows:

First stage H2 2 H2 H41 OH OH HOH 1 J31 (Lu Second stage The structure of a representative portion of this molecule is as follows:

H I LCH2.CH2.N.U112.0H2.I l.CH2.CHz.N.CH2.CH2.I I

H2 H2 JHQ HO OH HO 011 Ht OH Ha Ha H H2 ILICHLCHLI LCHLCH2.N.CH .CHQJ LCHLCHLI I (1H2 H2 CH2 H( OH H OH H2) 011 Hz H H2 H Hz P LCHLCHLP I.CH2.CHLI LCHQ.CHg.I I.CHg.CHz.l I

CH2 H2 [H2 HOOK HO OH H OH CH2 I I It is evident, therefore, that the essential reagents for preparing the thermosetting resins used in practicing the invention are (1) an alkylenepolyamine and (2) a poly-functional halohydrin such as a dihalohydrin, e. g. alpha-dichlorhydrin, dibromhydrin or di-iodhydrin or any of the corresponding monohalohydrins containing a second radical or group capable of reacting or promoting reaction with an alkylenepolyamine such as epichlorhydrin, epibromhydrin, epi-iodhydrin, di-epi-iodhydrin and the like.

The above reaction mechanism also suggests that more than one molecular proportion of the polyfunctional halohydrin should be present for each mol of tetraethylenepentamine if a thermosetting resin is to be obtained, and this has proven to be the case. When equimolecular quantities of epichlorhydrin and tetraethylenepentamine were reacted the product was not thermosetting and did not impart wet strength to paper; however the condensation product of two mols of epichlorhydrin or dichlorhydrin with one mol of the tetraethylenepentamine was thermosetting in character and produced paper of good wet strength. Further tests have shown that this ratio can be lowered to about 1.5:1 while obtaining thermosetting and wet strength-imparting properties, but for most practical purposes the 2:1 molar ratio marks the lower limit of the range wherein a commercially satisfactory wet strength resin is obtained. Larger proportions can be and. usually are employed to increase the speed of cure of the resin, the optimum being between about 2:1 and 3-: 1. When more than 4 mols of the poly-functional halohydrin are used for each mol of tetraethylenepentamine the resin is thermosetting and produces wet strength paper, but the syrups are less stable in character and must be manufactured and stored at higher dilutions to avoid premature gelation.

It will readily be understood by those skilled in the art of synthetic resin manufacture that polyamines containing a smaller number of amino groups than tetraethyl'enepentamine will form thermose'tting resins with correspondingly reduced molar proportions of poly-functional halohydrins; time, as shown in Example 7, good results are obtained when the ratio of epichlorhydrin to diamines or triamines is 2:1.

The thermosetting resins applied to paper or paper stock by the present invention are therefore prepared by reacting one mol of an alkylenepolyamine with at least 1.5 mols and preferably 2 or more mols of a polyfunctional halohydrin. This reaction is preferably carried out at temperatures below the boiling point of the mixture, usually not substantially higher than 60-70 C., in order to permit the use of relatively concentrated solutions while obtaining the resin in a hydrophilic or water-dilutable condition. Us'ually the halohydrin is added slowly to the alkylenepolyamine, which is preferably dissolved in water or a water-miscible solvent such as aqueous ethanol, at a rate such that the reaction temperature is maintained at about 50-55 C. The reaction product may then be maintained at 6-070 C, until an increase in viscosity is noted, indicating that the second stage of the abovedescribed reaction has set in, after which it is cooled and diluted with water if necessary to form a stable syrup. In some cases, and particularly where a dihalohydrin is being used, sufficient alkali such as sodium hydroxide, carbonate or phosphate should be added before or during the second stage of the reaction to neutralize the syrup by combining with any hydrohalide that is not taken up by the polyamine. This alkali addition also gives improved results when condensing a polyamine of relatively low molecular weight, such as ethylenediamine, diethylenetriamine or triethylenetetramine with several molecular proportions of a monohalohydrin such as epichlorhydrin. If desired the syrup may be subjected to a vacuum distillation after the first stage of the reaction is completed to remove any unreacted epichlorhydrin, dichlorhydrin'or other poly-functional halohydrin.

As has been stated, the alkylenepolyaminehalohydrin resin solutions can be applied to paper or other felted cellulosic products by tub application methods if desired. Thus, for example, preformed and completely or partially dried paper prepared from kraft, sulfite or rag, soda, sulfate or ground wood stock or any mixture thereof may be immersed in a 310% aqueous solution of the resin and impregnated with about 90-100% therof, based on the weight of the paper. The paper is then heated for about 1-4 minutes at temperatures of 2l2-300 F. or higher, or for shorter times at higher temperatures, whereby the paper is dried and the resin is cured to a water-insoluble condition. The resulting paper has greatly increased wet strength, and therefore this method is well suited for the impregnation of paper towels, absorbent tissue, cigarette paper and the like as well as heavier stock such as wrapping paper, bag paper and the like to impart wet strength characteristics thereto.

The preferred process of the present invention,

- however, takes advantage of the substantive properties of the halohydrin-alkylenepolyamine resins for hydrated cellulosic fibers. In practicing this process the resin in its uncured and hydrophilic or water-dilutable condition is added to an aqueous suspension of the paper stock, such as any of those enumerated above, in the beater, stock chest, Jordan engine, head box or at any other suitable point ahead of the papermaking wire or screen followed by forming the treated fibers into a felted product on the wire or cylinder. The felted product is then heated in the usual manner to dry'the paper or board, thereby curing the resin to its polymerized andwater-insoluble condition and imparting wet strength to the paper.

As is noted above, the thermosetting reaction products of alkylenepolyamines with poly-functional halohydrins impart substantial wet strength to paper and other products formed of felted cellulosic fibers when present therein in amounts of about 0.5-5% or more. The quantity of resin to be added to the aqueous stock suspension will depend on the degree of dry and wet strength desired in the finished product and on the per cent of resin retained by the paper fibers. Thus, for example, with a resin retention in the stock, 6% of resin should be added to produce a wet strength paper containing 3% of resin, based on the dry weight of the paper. The resin remaining in the white water can be re-used in treating further quantities of paper by employing a closed or partially closed or recirculating white water system; i. e., by using a part or all of the white water from the papermaking machine for preparing further batches of paper pulp suspension.

We have found that the uncured alkylenepolyamine-halohydrin resin contained in paper, whether introduced as a tub size or combined with the cellulosic fibers prior to sheet formation by adsorption in aqueous suspension, can be cured under acid, neutral or alkaline conditions by subjecting the paper to a heat treatment. This is of considerable importance, since it permits addition of the water-soluble resin to the paper stock suspension'in the beater or stock chest of a paper mill along with clay, alum, resin size, talc, calcium carbonate and other suitable sizing or loading materials. Even when large quantities of calcium carbonate are added, which cause the paper to be alkaline in character, the resin can be cured by heating between steam-heated rolls in the usual manner at temperatures of 2l2-350 F. or higher. The resin can also be used successfully with acidic materials; thus, for example, it has been applied to aqueous paper pulp in admixture with the melamine resin acid colloid described above and cured in the resulting paper without difiiculty.

The invention will be illustrated in greater detail'by the following specific examples. It should be understood, however, that although these examples may describe in detail some of the more specific features of the invention they are given primarily for purposes of illustration and the invention in its broader aspects is not limited thereto.

EXAMPLE 1 A solution of 37.8 grams (0.2 mol) of tetraethylenepentamine in 93 grams of water was prepared and epichlorhydrin was added with stirring at a rate such that the reaction temperature did not rise above 50 0. After 35.5 grams (0.6 mol) of epichlorhydrin had been added the stirring was continued at 50 C. until the product had become viscous. The resulting hydrophilic colloid was diluted with water to a 15% solution.

Bleached kraft paper pulp was suspended in water and refined 2. minutes, diluted with water to a fiber consistency of 0.6%, and divided into a number of samples. The above-described epichlorhydrin-polyalkylenepolyamine resin was added to all of these samples except one control. The pH of the resin-treated fiber suspensions was then adjusted to the values shown in the following table by adding hydrochloric acid or sodium hydroxide or, in sample No. 8, by adding aluminum sulfate, and the samples were made into handsheets on a laboratory papermaking machine. All the sheets were heated one minute at 230 F. to dry the paper 7 and cure resin, and sheets from each sainple were also given an additional cure of minutes at 260 F. The sheets were then tested for wet 8 to tetrathylenepentamine and the tensile strength is' in pounds per inch width.

and dry tensile strength.

In the following table the per cent resin 5 2 Strength MIT added is based on the dry weightof the knit Sample R200 Fold fibers; the per cent retained is based on the Dry Wet amount of resin added, and the basis weight is the weight in pounds of 500 sheets 24 inches by inches in size. The tensile strength was 5 1 626 measured on 4 x 0.5-inch strips, but is reported 602 as pounds per inch width.

H f Per Cent Resin B p Tensile Strength, lbs/inch D 0 2515 sample Stock Weight Added Retained Dry Wet ;Dry Wet .5 4.5 5.0 12 50.1 22.0 3.4 22.8 5.8 4.5 None 50.0 19.4 0.4 13.4 0.0

1 After additional heating.

These results show that good wet strength is EXAMPLE 4 obtained under both acid and alkaline conditions with the thermosetting epichlorhydrin-polyalkylenepolyamine resin. However, the retention and wet strength are better under alkaline conditions.

EXAMPLE 2 The folding endurance of paper prepared from stock suspensions pretreated with thermosetting epichlorhydrin-polyalkylenepolyamine resin is greatly increased over that of paper made from the same stock which contains no resin. This is shown in the following table, the paper being made from samples of the resin-treated stock described in Example 1 by the procedure described in that example.

Bleached kraft paper pulp was dispersed in water in a laboratory beater, beaten to hydrate the stock, refined 2 minutes in a Morden refiner and diluted to 0.6% fiber consistency. Three per cent of the epichlorhydrin-polyalkylenepolyamine resin of Example 1 was added followed by 100% of calcium carbonate filler, based on the dry weight of the paper fibers. The stock was then made into handsheets, some of which were given a second cure of 10 minutes at 260 F. in

addition to the regular heating for one minute at 230 F. The sheets were tested for resin content Tensile Tensile Per cent Resin Sample pH of Basis Strength MIT Strength No. Stock Weight Fold Added Retained Dry Wet Dry Wet 4. 5 3.0 38 47.8 24. 4 3. O 557 24. 8 5. 8 8.0 3.0 43 48. 9 25. 2 4. 4 624 25. 4 6.6 4. 5 1. 5 53 48. 2 24. 2 2. 8 604 25. 4 6. 4 4. 5 0. 75 76 49. 2 23. 6 2. 4 718 24. 0 4.6 Allim 0.75 57 47. 9 23. 2 2. 0 636 24'. 6 4. 4

4. 5 None 48. 7 22. o o. 0 388 21.8 0. 5

After an additional IO-minute cure at 200 F.

These figures also show that paper having good wet strength is obtained when even small quantities of the resin are used.

EXAMPLE 3 Following the procedure described in Example 1 a series of resin syrups was made from tetraethylenepentamine with varying molar ratios of epichlorhydrin and tested by adding 3% of the resin to an 0.6% water suspension of bleached kraft paper pulp, followed by forming the treated stock into handsheets, heating one minute at 230 F., and testing for dry and wet tensile strength and folding endurance. The results are given in the following table in which the heading and for dry and wet tensile strength in the usual Another advantage of the thermosetting polyalkylenepolyamine resins is the fact that they will function well in water containing large quan- Ratio means the molar ratio of epichlorhydrin tities of dissolved salts. In order to demonstrate this property, both the stock and recirculated white water were adjusted by the addition of sodium sulfate to give the desired sulfate ion concentration and 3% of resin, based on the dry weight of the stock, was added and handsheets were made and tested with the following results:

1 After additional IO-minute cure at 260 F.

EXAMPLE 6 A solution of 94.5 grams (0.5 mols) of tetraethylenepentamine in 492 grams of water was prepared and 161.2 grams (1.25 mols) of dichlorhydrin (ClCH2.CH(OH) .CH2Cl) was added slowly with agitation while maintaining the temperature below C. The syrup was then cooled to 10 C. and a solution of 51.5 grams of 97% NaOH in 150 grams of water was added. The mixture was agitated for 3 hours to initiate the formation of a thermosetting resin. The resulting resin syrup, having a pH of 8.0 and a solids content of 22%, was diluted with water to 10% solids and was stable for several weeks at this concentration.

The resin was added to aqueous 0.6% suspensions of bleached kraft paper pulp, using 3% of resin on the dry weight of the pulp, with and without the addition of aluminum sulfate. Handsheets were made from the treated stock and tested for dry and wet tensile strength in' the usual manner. The sheets containing the resin with no alum had dry and wet strengths of.

26.0 and 4.8 pounds per inch width, respectively.

The corresponding figures for sheets containing 3% alum along with the resin were 28.2 and 5.8 pounds.

EXAMPLE '7 Resin No. 1

To a solution of 25.75 grams of diethylenetriamine in 83.7 grams of water there was added slowly 57.9 grams of epichlorhydrin while maintaining the temperature at 50 C. When the initial condensation was complete 11.3 grams of trisodium phosphate was added and the solution was heated at 200 F. until an increase in viscosity indicated that the second stage of the condensation had been reached. This required about one hour; The resulting resin syrup was diluted with water to about 10% solids.

Resin No. 2

A solution of grams of ethylenediamine in 109 grams of water was prepared and 92.5 grams of epichlorhydrin was added slowly while cooling the mixture to C. After this addition the solution was heated to about 70 C., 18 grams of trisodium phosphate was added, and heating was continued to form a resin syrup which was cooled and diluted.

Resin N0. 3

To a solution of 36.5 grams of triethylenetetramine in 92 grams of water there was slowly added 55.5 grams of epichlorhydrin at 50 C. The resulting syrup was further reacted by heating to an increased viscosity and diluted to 10% solids.

Each of these resin syrups was added to an 0.6% suspension of kraft paper pulp in water, using 3% of the resin based on the dry weight of the pulp, and the treated stock was made into handsheets which were cured at 230 F. for one minute. Some of the sheets from each batch were givenan'additionalcure of 10 minutes at 260 F. The sheets were then tested for dry and wet tensile strength and folding endurance. In the following table the tensile strengths are given in pounds per inch Width of the paper.

. Tensile Strength if m gig ggfi Re n we MIT Fold Dry Wet Dry Wei:

What we claim is: i a

1. Paper having a uniform content of about 0.5-5% of its dry weight of a cured thermosetting alkylenepolyamine-polyfunctional halohydrin resin.

2. Paper having a uniform content of about 0.55% of its dry weight of a cured thermosetting resin, said resin being the condensation product of one molecular proportion of an alkylenepolyamine with at least 1.5 mols of a polyfunctional halohydrin.

3. Paper having a uniform content of about 0.5-5% of its dry weight of a cured thermosetting tetraethylenepentamine-polyfunctional halohydrin resin.

4. Paper impregnated uniformly with the heat-cured condensation product of one mol of a polyalkylenepolyamine with at least two mols of a dihalohydrin.

5. Paper impregnated uniformly with the heat-cured condensation product of one mol of a polyalkylenepolyamine with at least two mols of an epihalohydrin.

6. Paper impregnated uniformly with the heat-cured condensation product of one mol of an ethylenepolyamine with at least two mols of an alphadihalohydrin.

7. Paper impregnated uniformly with the heat-cured condensation product of one mol of an ethylenepolyamine with at least two mols of an epihalohydrin.

8. Paper impregnated uniformly with the heat-cured condensation product of one mol of a polyethylenepolyamine with at least two mols of alphadichlorhydrin.

9. Paper impregnated uniformly with the heat-cured condensation product of one mol of a polyethylenepolyamine with at least two mols of epichlorhydrin.

10. Paper carrying an alkaline filler and having a uniform content of a cured thermosetting alkylenepolyamine-polyfunctional halohydrin, the amount of said resin being about 0.55% based on the dry weight of said paper exclusive of the weight of said filler.

11. Paper according to claim 10 in which the filler is calcium carbonate.

12. A process for the production of cellulosic products of increased wet strength which com- I! curing the resin to its heat-set and water-insoluble condition by heating said waterlaid product for about 1 to 4 minutes at temperatures between 300 and 212 F. and thereby forming a bond of cured resin between the cellulosic fibers thereof.

13. A process according to claim 12 in which the resin is a polyethylenepolyamine-epichlorhydrin resin.

14. A process for the production of wet strength paper which comprises adding to an aqueous suspension of cellulosic paper stock an uncured thermosetting alkylenepolyamine-polyfunctional halohydrin resin, adsorbing about 0.1% to 5% of said resin on said paper stock, forming the stock so treated into a waterlaid sheet, and curing said resin to its heat-setand water-insoluble condition by heating said sheet for about 1 to 4 minutes at temperatures between 300 and 212 F. and thereby forming a bond of cured resin between the cellulosic fibers thereof.

15. A process according to claim 14 in which the resin is a polyethylenepolyamine-epichlorhydrin resin.

JOHN H. DANIEL, JR. CHESTER G. LANDES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,977,252 Stallmann Oct. 16, 1934 2,020,646 Hornstein Nov. 12, 1935 2,136,928 Schlack Nov. 15, 1938 2,325,302 Britt July 27, 1943 2,338,602 Schur Jan. 4, 1944 2,343,095 Smith Feb. 29, 1944 2,345,543 Wohnsiedler Mar. 28, 1944 2,354,574 Carson July 25, 1944 2,402,469 Toland et al June 18, 1946 2,407,376 Maxwell Sept. 10, 1946 2,414,289 Ericks Jan. 14, 1947 OTHER REFERENCES 25 pages 263-269. 

1. PAPER HAVING A UNIFROM CONTENT OF ABOUT 0.5-5% OF ITS DRY WEIGHT OF A CURED THERMOSETTING ALKYLENEPOLYAMINE-POLYFUNCTIONAL HALOHYDRIN RESIN. 